1. Field of the Invention
The present invention relates to an additional-data embedding apparatus and additional-data embedding method for embedding additional data into original-work data, such as image data, using so-called digital watermarking.
2. Description of the Related Art
With the recent popularity of recording media such as CDs (Compact Discs), MDs (Mini Discs), and DVDs (Digital Versatile Disks), and the Internet, there has been a problem in that copyright is violated by illegally copying someone's work or digital data, which is said to be easily duplicated or altered. In order to address the problem, for example, a so-called digital watermark (or digital data embedding or digital data hiding) technique has been proposed, which is a technique to embed additional information including copy control information for digital data, such as “COPY ONCE” or “NEVER COPY”, and copyright information indicating the copyright holder into original data including image data for still images or moving pictures, and audio data for sound or music.
In digital watermarking, for example, when additional data is embedded into image data, the additional data is invisible while maintaining the quality of the original image data. When additional data is embedded into audio data, the additional data is inaudible while maintaining the quality of the original audio data.
Additional data embedded as digital watermark information into original data is detected by a recording device when the original data is copied. In digital watermarking, therefore, various information such as data distributing routes, the presence or absence of a license, and copy control information can be detected based on the detected additional data, thereby preventing the original data from being illegally copied.
As requirements for digital watermarking, first, the embedded additional data should be imperceptible (e.g., invisible, inaudible, etc.); and, second, if data having additional data embedded therein is artificially modified by a third party, the embedded additional data should be robust. These requirements are conflicting. In general, such a digital-watermark embedding technique using information processing technology is designed so that additional data is strongly embedded into a perceptible region of the original data and is weakly embedded into an imperceptible region of the original data.
For example, for digitally watermarking a still image, additional data WM having a value of ±1, and luminance for image data I are summed to obtain watermarked image data I′. For digital watermarking, furthermore, as expressed in the following Equation (1), the additional data WM may be multiplied by α·I, which depends upon the pixel value into which the additional data WM is to be embedded, thereby changing the embedding strength of the additional data WM:I′=I+α·I·WM  (1)This uses the property of the human visual system that, in a region having a higher pixel luminance, a change to that pixel is less perceptible on a display device.
U.S. Pat. No. 5,748,763 assigned to Digimarc Corporation discloses a digital watermarking technique which can be implemented by a digital-watermark embedding apparatus 100 shown in FIG. 9. The digital-watermark embedding apparatus 100 includes two modulators 101 and 102, and an embedder 103 having an adder. The digital-watermark embedding apparatus 100 supplies to the first modulator 101 both original image data I and additional data WM to be embedded, where the embedding strength of the additional data WM for each region of the original image data I, called a local parameter, is computed with functions according to the luminance of a region into which the additional data WM is to be embedded. The additional data WM is then modulated by the modulator 101. The digital-watermark embedding apparatus 100 supplies the additional data WM modulated by the modulator 101 to the second modulator 102, where the embedding strength of the additional data WM for the overall original image data I, called a global parameter, is determined. The additional data WM is then modulated by the modulator 102. The digital-watermark embedding apparatus 100 embeds the additional data WM, which is level-controlled by the modulator 102, into the original data using the embedder 103, and outputs the image data I′ having the additional data WM embedded therein.
In the above technique, therefore, depending upon the properties of the image data I, additional data WM is modulated by the modulator 101, and the embedding strength of the additional data WM with respect to the overall image data I is determined by the modulator 102, after which the additional data WM is embedded.
In the above-described technique, if the embedding strength of the additional data WM is changed depending upon the original image data I, calculations such as multiplications and functional equations must be performed, thus increasing the power consumption of a CPU (central processing unit) when the apparatus 100 is implemented by software, or increasing the size and complexity of the circuit if the apparatus 100 is implemented by hardware. In particular, if the embedding strength of the additional data WM is changed depending upon the original image data I, this technique would only need to store, as a reference table, the embedding strength of the additional data WM depending upon the original image data I in a storage unit without using either the global parameter or the local parameter; however, in fact, the calculation is performed using both the global parameter and the local parameter, thus increasing the complexity of the processing.